1. Field of the Invention
Metallic tubular heat exchangers must pass heat through the metallic wall and the two exchanging media film resistances. In this indirect exchange, the two media often offer unequal resistance to heat flow. Boilers, superheaters, economizers and air-cooled exchangers have greater shell side resistance to heat flow than the tube side. So, designs commonly employ various designs of extended surface on the outside of tubes. Materials of construction and methods of attachment depend on the intended service conditions. But very high unit area gains are manufacturable in transverse, helically wrapped fins. Area ratios of fin surface to bare tube surface are commonly 8:1 to 16:1.
Generally, the higher the extended area ratio the more compact and cost effective the heat exchanger. There is a manufacturing vs. design interplay between fin parameters to determine good fin combinations. But more fins per unit length and taller fins yield ever increasing extended area ratios. Limits are imposed by the fin efficiency as well as the manufacturing process.
Many fin surfaces are utilized in single phase, gas heat transfer. Investigation into the structure of fin surfaces teach ways to increase heat transfer rate, increase unit surface area, and promote surface self cleaning- Segmented or discontinuous fins interrupt gas boundary layer build-up and have greatly increased heat transfer rates over solid, smooth fins. Again compared to solid fins, the manufacturing process becomes much easier for segmented fins so that taller fins are producible, yielding higher unit surface areas.
Fin surfaces used in fluid phase change applications benefit from more complex geometry. Here the fin structure benefits by having multiple nucleation sites. The sites encourage more continuous wetting and very small pressure gradients.
2. The Prior Art
There have been many improvements in fin structures, requiring several modifications in the manufacturing process. Although these improvements are interrelated, there is a hierarchy. The segmented fin is an improvement in heat transfer rate and may be as an independent fintube or in a composite HVAC (heating, ventilating, air-conditioning) exchanger.
Patent searches were conducted and the following listed patents are deemed to be broadly pertinent:
______________________________________ Patent No. Inventor Issue Date ______________________________________ British 906,282 Carr 09/19/62 3,183,970 Worley 05/18/65 4,040,479 Campbell, et al. 08/09/77 Japanese 56-130598 Jukogyo, et al. 03/17/80 4,211,276 Itoh 07/08/80 4,227,572 Harlan 10/14/80 4,480,684 Onishi 11/06/84 4,538,677 Bodas, et al. 09/03/85 ______________________________________
U.S. Pat. No. 4,480,684 (Onishi) taught up to 20% heat transfer rate improvements by using discontinuous and convoluted fin surfaces in an HVAC exchanger. Likewise, U.S. Pat. No. 3,183,970 (Worley) taught long radial depression in fin sides, while U.S. Pat. No. 4,227,572 (Harlan) taught fin edge tears, both increasing rates in an independent fintube. U.S. Pat. No. 4,040,479 also taught rate improvement by edge tears for nucleate boiling sites.
Prior art teaching increased unit area include British Patent No. 906,282 (Carr) wherein fin surface grooves and serrations brought the areas up by a factor of two (.times.2), and U.S. Pat. No. 4,211,276 (Itoh) wherein film surface sheet metal was doped to affect area gain depending on dope contact angle in a HVAC style exchanger. At a dope contact angle of 60.degree. the gain was two (.times.2). Lower fin surface fouling through increased velocities was taught by U.S. Pat. No. 4,538,677 (Bodas) and Japanese 56-130598 (Jukogyo).